An automated multi-step variable-speed transmission of the type mentioned above is understood to be an automated variable-speed transmission, an automated dual-clutch transmission or a planetary automatic transmission. The term automated multi-step variable-speed transmission also covers an automatic transmission whose transmission ratio as such can be varied continuously, but in which change-over gearing enables the rotation direction of the transmission output shaft to be reversed and in which, by appropriate control of transmission ratio changing means, shifting between transmission ratio steps firmly predetermined by programming means is also possible.
An automated variable-speed transmission of countershaft design usually has its input shaft connected to the drive engine via a single controllable friction clutch, i.e. one that can be engaged and disengaged automatically, such as a passively engaging single or multiple disk dry clutch or an actively engaging disk clutch, such that the friction clutch fulfills the functions both of a starting element and of a shifting clutch. In the case of an automated dual-clutch transmission consisting of two part-transmissions, each with an input shaft, the two input shafts can each be connected by a respective controllable friction clutch to the drive engine. In contrast, a planetary automatic transmission is usually connected with its input shaft to the drive engine by a hydrodynamic torque converter that forms a starting element. However, other combinations of multi-step variable-speed transmissions and starting elements are also possible, for example a combination comprising an automated variable-speed transmission with an upstream hydrodynamic torque converter, or one comprising a planetary automatic transmission with an upstream friction clutch.
In a motor vehicle whose drivetrain comprises such a combination of a multi-step variable-speed transmission and a starting element, when the drive engine is running and the drivetrain is disengaged, i.e. when the multi-step variable-speed transmission is in its neutral position and/or the starting element is disengaged, there is, in general, a risk that without the driver wanting this the motor vehicle may begin rolling because a gear is engaged or because it is on a gradient, which can lead to dangerous situations both for the vehicle's occupants and for other, nearby traffic.
For example, it is possible for a motor vehicle equipped with an automated multi-step variable-speed transmission and a passively engaging friction clutch, when the drive engine is running, the friction clutch is disengaging and a gear is engaged, to start off autonomously, if a defect develops in the clutch control system as a result of which the friction clutch engages unintentionally. To avoid this, for example in DE 199 26 697 A1 a device and method are proposed by virtue of which, when the motor vehicle is stationary, the drive engine is running, the friction clutch is disengaged and a gear is engaged, the multi-step variable-speed transmission is shifted to its neutral position, i.e. the engaged gear is disengaged, when it is detected that the motor vehicle has been in a condition in which it is not ready to drive for a predetermined period of time.
In a similar method described in DE 102 58 834 A1, in the case of a drivetrain designed in principle in the same way it is provided that when the motor vehicle is at rest, the drive engine is running, the friction clutch is disengaged and a gear is engaged, the multi-step variable-speed transmission is shifted to neutral if it is detected that one of the vehicle's doors has remained open for a predetermined time. In this way undesired starting of the motor vehicle can be avoided, which might result either from a defect in the clutch control system or due to inadvertent actuation of a driving operation element, in particular the accelerator pedal, that causes the friction clutch to engage.
However, undesired rolling of the motor vehicle due to an uphill or downhill road gradient cannot be prevented in this way. Thus, when the drivetrain is disengaged the driver must engage a parking brake or pull it up hard enough to keep the motor vehicle safely immobile. But if the parking brake has not been engaged or not pulled up hard enough, for example because the driver has underestimated the gradient and has therefore not engaged the parking brake or not pulled it up hard enough, or because the driver or some other occupant of the vehicle has inadvertently released the parking brake, then the motor vehicle can roll downhill uncontrolled. It is true that if the driver in such a situation is in his seat and notices the rolling, he can stop the uncontrolled rolling by actuating the service brakes or by engaging a gear by actuating a shift operation element and sometimes by actuating the accelerator pedal. But for this, the driver must be present in his seat and must react quickly, which is not always certain.